Pb—Sn solder has been used for bonding of metals from long in the past. Recently, Pb—Sn solder has been widely used for soldering of electronic equipment and particularly for soldering of electronic parts to printed circuit boards. Pb—Sn solder has a melting point of 183° C. at a eutectic composition (Pb-63Sn), and soldering can be carried out at a temperature of 240° C. or less, so it does not produce thermal effects on electronic parts or printed circuit boards. In addition, a Pb—Sn solder near this eutectic composition has the excellent properties that it has excellent wettability and does not cause soldering defects. Therefore, it has contributed to the reliability of electronic equipment.
However, as awareness of the environment has increased in recent years, restrictions on the use of lead have developed through RoHs regulations. As a result, the use of Pb—Sn solder, which contains lead, is being regulated, and so-called lead-free solder which does not contain lead is being increasingly used.
Lead-free solder is a solder alloy having Sn as a main component. Typical examples of lead-free solders which have been used in the past include Sn-3.5Ag (melting point of 220° C.), Sn-3Ag-0.5Cu (melting point of 217-220° C.), Sn-5Sb (melting point of 240° C.), Sn-9Zn (melting point of 199° C.), and Sn-0.7Cu (melting point of 227° C.). Each of these lead-free solders has a Sn content which is quite high compared to that of a Pb—Sn solder.
Among these lead-free solders, Sn-3.5Ag and Sn-3Ag-0.5Cu have superior solderability compared to other lead-free solders. However, they contain a large amount of expensive Ag, so they are not so suitable for soldering in fields where there is intense price competition.
Sn-5Sb uses inexpensive raw materials, but due to its melting point which is high, its soldering temperature necessarily becomes high, and thermal effects on electronic parts and printed circuit boards become a problem. Therefore, its uses are extremely limited.
Sn-9Zn has a melting point of 199° C., which is close to that of a conventional Pb—Sn eutectic solder. Therefore, it has no problems of thermal effects on electronic parts or printed circuit boards. However, Sn-9Zn not only has poor wettability, but peeling of soldered joints when they are subjected to an external impact has been reported. This peeling is due to a large difference in the ionization tendency of the Zn in the solder and the Cu present in portions to be soldered. As a result, when moisture in the periphery of soldered joints condenses, electrochemical corrosion caused by a local cell action is produced, and it becomes easier for peeling of soldered joints to take place.
Because Sn-0.7Cu is inexpensive and it has a relatively high surface gloss, it is used by preference for plating of inexpensive parts to be soldered on printed circuit boards (such as connectors). However, its wettability is not sufficient for use in soldering of electronic equipment, so there is a need for an improvement in its wettability.
Below-identified Patent Document 1 proposes adding a small amount of P or P and Ge to Sn-0.7Cu to improve its wettability. The Sn—Cu based solder disclosed in that patent document may contain at least one element selected from Ag, Sb, Ni, Co, Fe, Mn, Cr, Mo, Bi, In, and Zn in order to improve its various properties. There is an example containing Bi in which 2 mass % of Bi is added.
Electronic equipment and particularly portable electronic equipment such as mobile phones, notebook computers, and cameras are used even in extremely cold regions at high latitudes. In such extremely cold regions, the temperature sometimes reaches an extremely low temperature such as −20° C. or below, and soldered joints in portable electronic equipment which is sometimes taken outside and used there is exposed to such extremely low temperatures. It has been reported that when lead-free solder is exposed to extremely low temperatures, it may become brittle and break. Furthermore, as portable electronic equipment is sometimes dropped and receives an impact, soldered joints in portable electronic equipment need to have good impact resistance.
Embrittlement of lead-free solder in extremely cold regions is generally caused by tin pest. Tin pest is a phenomenon in which white tin (β-Sn) which is a tetragonal crystal and which is soft and highly ductile at room temperature undergoes an allotropic transformation at low temperatures into a gray cubic crystal (α-Sn, gray tin) which has no ductility and is extremely brittle. This phenomenon has been known from long in the past. The transformation temperature from β-Sn to α-Sn is approximately 13° C. However, due to the effect of supercooling, tin pest does not actually develop until the temperature reaches −20° C. or below, and it becomes marked around −40° C. As the use of electronic equipment in extremely cold regions where the air temperature falls below −20° C. increases, a concern has developed of embrittlement and peeling of soldered joints caused by tin pest.
Below-identified Patent Document 2 proposes adding (1) Pb, (2) Pb+one or both of Bi and Ag, or (3) Bi+Ag in an amount of 150-900 ppm (=0.015-0.09%) in order to prevent the occurrence of tin pest in a solder alloy having tin as a main component. The type of solder alloy can be any of various compositions such as Sn—Cu based alloys, Sn—Ag—Cu based alloys, Sn—Sb—Ag based alloys, and Sn—Zn—Bi based alloys.
Although not intended to suppress the occurrence of tin pest, Patent Document 3 proposes a lead-free solder consisting essentially of, in mass %, 0.1-5.0% of Bi, 0.1-5.0% of Ag, 0.1-3.0% of Sb, 0.1-5.0% of Cu, 0.001-0.01% of P, 0.01-0.1% of Ge, and a remainder of Sn, and Patent Document 4 proposes a Sn—Cu—Ni alloy which contains, in mass %, 0.01-0.5% of Ni, greater than 2% and at most 5% of Cu, and optionally at least one element selected from Ag, In, Zn, Sb, Bi, Ge, and P in an amount of at least 0.01%. In the examples in these patent documents, Bi is added in a large amount of at least 3 mass %.    Patent Document 1: JP 2003-94195 A    Patent Document 2: JP 2006-212660 A    Patent Document 3: JP H10-225790 A    Patent Document 4: JP 2006-26745 A